c.stenson1

i have a 93 fd3s all standard but have a back fire from hell, i went to work one day about three months ago all was fine good boost the lot, but when i got in my car after a days work i drove up the road and was deafend by a back fire and a MASSIVE loss of power. i got home after constant back fireing in every gear (it only does it when i give it some gas, i can drive like my grandma and its ok, but who wants that with an rx) and started to look for the problem, i found a few check valves had split open and a few of the solanoid valves were broken, so i replaced the check valves with viton ones and got other solanoid valves, but it was still the same, i then took it to a garage for a diagnostic check, it came up with a fuel relay so i replaced that but with no luck. it isnt in limp mode as i have done the battery off and tapped the brake pedal, no ic pipes are craked, all the little vaccum pipes are ok, ive changed the spark plugs but it is still the same. what else could it be, it just seems like something fell off or snapped when cooling down that day, but i cant see what, can anyone help, cheers phil.

MR_Rick

iTrader: (7)

check your MAP (boost sensor) hose. It might be off.

mibad

1. Compression Check.

cbf246

^^^^ agree. my FD had very low compression, and it backfired like crazy. people liked it cause it spit out blue flames, but it wasnt good for the car at all.

WaLieN

You don't just tap the pedal, you have to hold it down for 30 seconds w/ the ground disconnected from the battery.

c.stenson1

thanks, would low compression cause it to hold back and back fire if i give it some gas? but drive ok if i drive slow. i did the resetting of the ecu and im sure ive done it for a reasonable amount of time, but i will do it for 1 munute just to make sure with the ground cable off, then i will know i can eliminate that. where is the map (boost sender) hose that i need to check, is it the little black box on right of the fire wall with a vaccum pipe coming off it.

Sgtblue

iTrader: (16)

Yep. Although I think if it were the sensor, you'd notice problems with the idle first.
Your symptoms are similar to what my car did when my Intake Air Thermosensor went bad, but it throws a CEL. Maybe I missed it, but I don't recall any mention of one.

DamonB

I agree with the original fuel pump relay diagnosis. It sounds like the fuel pump is not being switched to high speed or fuel pressure is remaining too low when the pump switches to high speed. This is why the car runs perfectly fine at idle and when driving slowly but the instant you get on the gas hard the car will bog and pop constantly. You let off the gas and drive slowly and all is normal again. Since the fuel pump cannot get to high speed the engine isn't getting enough fuel to maintain power so it stutters and pops. You could actually remove the fuel pump relay and drive the car and it will act exactly as you describe. The fuel pump relay doesn't turn on the pump, it only switches the pump to high speed.

To test the fuel pump circuits put a jumper across the fuel pump resistor. This will make the pump operate at full speed 100% of the time. If your car drives fine with the resistor jumped then you know your problem is in the circuitry that controls the fuel pump. If the car still doesn't drive fine measure the voltage at the fuel pump itself. With the fuel pump resistor jumped you should be within a couple volts of battery voltage. If volatage is still low with the fuel pump resistor jumped this also indicates a wiring issue. It won't take five minutes to try this.

I highly doubt your engine has any issues. As soon as anyone has any kind of drivability problem everyone tells you it's blown Your engine is not blown.

c.stenson1

thanks damon, i forgot to mention that i did a compression and all four were 8.5 so like you said my engine is ok. what you said makes sense about the fuel pump i will do that but could you tell me how to locate it.

DamonB

The fuel pump resistor is an aluminum box mounted to the side of the engine bay just in front of the power brake booster. If you have cruise control you won't see it because it's hidden underneath.

Unplug the single two wire plug on the fuel pump resistor and then jam a wire across the terminals of the plug on the engine harness side. This bypasses the fuel pump resistor and makes the pump run at full speed at all times, regardless of the fuel pump relay working or not.

The fuel pump itself is in the gas tank under the shiny aluminum cover in the trunk. You don't need to remove it to check voltage though. Just unplug the large white connector under the shiny cover and measure the voltage from there. You'll have to check the FSM to see what wire is what. I don't remember off the top of my head.

c.stenson1

thanks very much, i will try that tomorrow as i am off out for the night now, i will let you know what happens tomorrow at about the same time, thanks once again for your help, phil in the uk.

c.stenson1

hey damon, i have just took a look in the engine bay before im off out, but im not sure where you mean my car is rhd, i can see a alluminium plate thing with fins on the right near the abs and a box on the left side of the engine bay with fins but they both have one plug with a lot of wires none with two wired plugs coming from them. can you direct me in real lame turms, which side of the engine bay is it and what does it look like. sorry for being a little thick, phil.

DamonB

^ Sorry; I didn't realize you were rhd. I wouldn't suspect the fuel pump resistor to be in a different place. On US cars if standing in front of the car the resistor will be bolted onto the far right side of the engine compartment on the "floor" just above the frame rail. It would be under the cruise control and the brake master cylinder on a US car. There will be one connector with 2 wires attached to it.

If it's not there I really don't know where it would be on a rhd car

c.stenson1

thanks very much ill let you know, must dash phil.

c.stenson1

hey, so i looked around the engine bay and the only thing i could find is this, is it the fuel pump resistor, its alluminium, its under ther brake power servo on the floor side of the engine bay and it has a plug with two wires, one is blue i cant make out the other. when i unplugged it the engine stalled after one or two seconds. is this a thumbs up or not.

DamonB

Sounds like you found it. Did you try putting a jumper across the plug yet?

c.stenson1

i tryed to put a picture on aswell but its to big, are there fins that run along the top of the resister. i did the jumper across and its the same, so ill test the voltage at the pump. cant i just run a long wire from the positive to the pump to make it run 100%.

DamonB

Sure, but it would run as soon as you make connection so be prepared to drive it that instant. If the car still runs crappy after doing that you can safely rule out any fuel pump wiring problems.

Even after replacing the fuel pump relay you could have issues with that circuit. The ecu only knows if the relay coil is present or not, it doesn't know if the wiring to and from the contacts is sound. You can remove the fuel pump relay and put a jumper across the fuel pump contacts as well as this will also force the pump to run at battery voltage. This will work fine for troubleshooting the fuel pump circuit with the car idling but if you try to drive the car hard the ecu will notice the relay isn't there and go into limp mode.

It's possible something else it tripping the ecu into limp mode but I can't think of anything that would cause that without also giving a check engine light. Are you no longer getting a check engine light since replacing the fuel pump relay? Did you verify that the old relay is in fact bad?

c.stenson1

ok thanks very much for your help. i will play about tomorrow and hopefully solve it with a long piece of wire. if i do that and its still got no power and back fireing could it be a solanoid valve or high fuel pressure blowing the spark out.
i have never had a check light an the dash, i see what you mean i could reset then drive hard and the ecu will go straight back in limp mode and have no power with back fireing. i have a few more ideas now chatting to you if i dont succeed i will post another thread.

DamonB

Your description doesn't sound like an ignition problem, especially since you got a diagnostic reading stating the fuel pump relay is bad and the circuit that relay controls can cause the exact symptoms you have.

Personally I would not even entertain any other possibilities until I knew with absolute certainty that the fuel pump itself and its wiring are working as they should. No sense in looking elsewhere until you have reason to. Troubleshoot one thing at a time, one step at a time.

I had the ground connection inside the tank for the fuel pump go bad once. If you find you have battery voltage at the connector under the shiny cover in the trunk it wouldn't hurt to go ahead and pull the cover off the fuel tank to be certain there is the same voltage at the fuel pump motor itself. That will only take a few more minutes to do.

c.stenson1

will do all that tomorrow, i will come to a conclusion on the pump and the wiring for sure. thanks once again for you help, ill be back to 100% soon, phil.

Ledfoot

iTrader: (1)

although the extra heat from being too lean can cause pre-ignition and i believe backfiring.......to me this sounds more like an ignition problem. You say it backfires when you step on the gas, i'm assuming it's backfiring from the exhaust. This could be that your ignition is not advancing with acceleration. Normally if the ignition is too advanced a backfire will occur on the intake, if too retard on the exhaust.

DamonB

The car would then pop and carry on but still have power available. His car falls completely flat on its face the exact instant the ecu switches the fuel pump to high pressure because there is not enough fuel present. If it were an ignition problem the car would have better performance than it does. With an ignition problem the car would feel like it wants to go, it will just lug and hold back. What he has is an instant loss of power that will throw you forward in the seat. I've experienced it.

The giant red flag waving in the breeze is the fact that the diagnostic computer said there is an issue with the fuel pump relay which controls switching of the fuel pump to high speed. His symptoms match exactly with this being the problem.

To feel what he's feeling remove the fuel pump relay and drive around the block. The car will seem perfectly normal until you load the engine hard enough and then all hell breaks loose as the car instantly looses all power and rapidly backfires out the exhaust. You let off the gas and it's fine. You accelerate slowly up to any speed and it's fine. You get on the gas hard and it will again loose all power and rapidly backfire out the exhaust.

Kaotic Dan

I also believe its ignition related. If the fuel is not ignited in the chamber it will exit out the exhaust and ignite thus causing a backfire. I don't understand how backfiring would be caused by running too lean from a fuel problem, as backfires are caused by fuel igniting in the exhaust. For example when the ECU is in limp mode it retards the timeing under boost, causeing sputting and backfiring with anything more than partial throttle, which sounds very similer to what is bieng described.

c.stenson1

hey good news, i did the test with the fuel pump, my long piece of wire and as soon as i got a positive to the pump i heard it go faster so i got in and tryed it, i was upset as it was still the same, but wait i havent finnished yet. i procceeded to look in the engine and found the fire wall cable had come loose, i also found that i have an uprated fuel regulater, so i tweaked it down till it tryed to stall and then back a quater of a turn. i then did the resetting of the ecu and this time i did it for 40 seconds and tapped it 10 times. when i started it an gave it some gas, OH MY GOD SHE IS BACK. i think that it did something when i messed about with the fuel pump, and thanks to damen i can reset the ecu propely.